Alternative splicing of the chicken c-myb exon 9A

The c-myb gene products are thought to be regulators of cellular replication and of differentiation and heterogeneity may underlie their multiple functions. To investigate the possible existence of heterogeneity we have examined the chicken c-myb mRNAs by Northern blot analysis and polymerase chain reaction amplification of cDNAs (RT-PCR). Northern blot analysis with the c-myb cDNA clone pSG3, which contains the entire open reading frame (ORF) plus 500 base pairs of 3' untranslated sequences (Gerondakis & Bishop, 1986), and genomic probes revealed c-myb RNA species of 4.3 kb in addition to the major 4.0 kb species. The 4.3 kb c-myb RNA contained the alternatively spliced exon 9A which is highly conserved and has also been detected in a minor 4.3 kb alternatively spliced c-myb mRNA in murine and human cells. Sequencing of the avian exon 9A revealed 360 bp exon homologous to that found in murine and human mRNAs, which contains three highly conserved sequence regions shared by all three species. RT-PCR demonstrated usage of exon 9A in five hematopoietic tissues and revealed an additional splicing variant which used the 3' portion of exon 9A. Northern blot analysis using splice site-specific oligonucleotide probes spanning the two splice junctions between exon 9 and 9A revealed four additional c-myb RNAs of 4.4 kb, 2.2 kb, 2.0 kb and 1.4 kb.